This invention relates to a scroll-type positive displacement apparatus, and more particularly to a scroll-type positive displacement apparatus which can be employed as a vacuum pump.
A scroll-type positive displacement apparatus (hereinunder referred to simply as a scroll-type pump) is a form of rotary pump in which the suction and compression chambers of the pump are defined by two interfitting scroll-shaped members, which are commonly referred to simply as scrolls. Generally, the scrolls each comprise a flat end plate and a spiral wall (commonly referred to as a wrap) which extends perpendicularly from the end plate. The two scrolls are disposed with their end plates parallel to one another and the spiral wraps interfitted so that the surfaces of the end plates and the spiral wraps define a plurality of spiral chambers, which serve as compression chambers and suction chambers. When the scrolls are rotated with respect to one another, the volumes of these chambers continuously vary and a fluid which is introduced into the chambers is transported either towards or away from the centers of the scrolls, depending on the direction of rotation. If the fluid is transported towards the center, it is compressed, while if it is transported away from the center, it is expanded.
Scroll-type pumps can be divided into two large classes. In one class of scroll-type pump, one of the scrolls is maintained stationary while the other scroll is orbitted about the center of the stationary scroll while being restrained from rotating on its own axis. In the other class of scroll-type pump, both of the scrolls rotate on parallel, nonaligned axes. With the first class of pump, it is necessary to provide counterweights to balance the moving scroll as it orbits. In the second class, however, each scroll undergoes balanced rotation on its own axis, so no counterweights are needed, and higher rotational speeds and higher pump capacities can be achieved.
The biggest problem which is encountered with scroll-type pumps is leaks between adjoining compression chambers. On account of the complicated shape of the spiral wraps of the scrolls, it is extremely difficult to maintain a high pressure differential between the suction and discharge sides of such a pump. Therefore, while there are a number of applications for which scroll-type pumps are suitable, it has not yet been possible to use them as vacuum pumps.